Power supply units are installed in a range of appliances including high frequency heating equipments for home use. Existing power supply units are generally heavy and large, and there is an increasing demand for smaller and lighter versions. Accordingly, development of smaller, lighter, and more inexpensive power supply units using a switching power supply are now being actively studied for use in a wide range of fields. There is also a demand for smaller and lighter power supply units for driving the magnetron in high frequency heating equipment for cooking food using magnetron-generated microwaves.
A switching power supply converts AC power or DC power to AC power with different frequency and voltage or DC power with different voltage. This is accomplished using semiconductor switching devices such as transistors and thyristors. In other words, the switching power supply is used for electric power conversion. Since the switching power supply operates semiconductor switching devices at high frequency, the reduction of switching loss is of key technological importance. In particular, switching power supply units used in high frequency heating equipment for home use may convert electric power at rates of above 1 kW. Reduction of switching loss is also important with respect to energy conversion.
A resonance circuit system, a configuration advantageous for reducing switching loss, is therefore employed in high frequency heating equipments. This circuit system is called the single switching element voltage resonance circuit, and it is a system designed to moderate voltage incline by applying the voltage in sine waveform to a semiconductor switching device utilizing the effect of a resonance circuit to reduce the switching loss.
However, the single switching element voltage resonance circuit of the prior art has the following weaknesses.
First, the voltage applied to the semiconductor switching device rises due to the effect of the resonance circuit. This may require the semiconductor switching device or related electrical components to be configured to a higher withstand voltage, resulting in a larger and more expensive power supply unit.
Second, although the ON time of the semiconductor switching device is settable, the OFF time is a function of the behavior of the resonance circuit and is not adjustable as required. This reduces the flexibility of control of the single switching element voltage resonance circuit. More disadvantages are caused in this respect, which will be described below while describing the single switching element voltage resonance circuit of the prior art in more detail.
FIG. 21 shows a circuit diagram of a power supply unit for driving a magnetron in high frequency heating equipment of the prior art.
FIG. 21 shows a power supply circuit for driving a magnetron which is powered from the AC power supply. Looking at FIG. 21 from the left, a full wave rectifier converts AC power supply from AC voltage to DC voltage, and the voltage V.sub.DC is applied to a circuit where the semiconductor switching device is connected in series with a parallel circuit comprising a capacitor and leakage transformer (a transformer with magnetic connection lower than 1 between the primary coil and secondary and tertiary coils due to leakage of magnetic flux). This semiconductor switching device operates at high frequencies. Here, an IGBT (Insulated-gate bipolar transistor) is employed as the semiconductor switching device. The leakage transformer and capacitor connected in parallel form a resonance circuit.
If a driving signal V.sub.G is applied to an IGBT gate for turning on the IGBT, a current I flows to the IGBT, passing through the primary coil of the leakage transformer. This is the period T.sub.1 in the waveform shown in FIG. 22A. When the IGBT turns off after the time T.sub.ON, the current starts to flow instead toward the capacitor and resonance is established. This is the period T.sub.2. FIG. 22B shows the waveform of the driving signal for IGBT. The energy W.sub.L of the leakage transformer can be defined: EQU W.sub.L =(LI.sup.2)/2 (Equation 1)
where I is the current and L is the inductance of the leakage transformer.
The current I can be defined: EQU I=V.sub.DC T.sub.ON /L (Equation 2)
where V.sub.DC is the voltage of the capacitor, i.e. the DC supply voltage.
When resonance starts, the above energy is transferred to the capacitor, establishing the following equation: EQU WL=(CV.sup.2)/2+W.sub.MG (Equation 3)
where C is the capacitance of the capacitor, V is the voltage of the capacitor, and W.sub.MG is the energy consumed in the rectifier and magnetron connected to the secondary coil side of the leakage transformer.
After energy is transferred to the capacitor, energy begins to be supplied from the capacitor to the leakage transformer, and resonance continues while attenuating as shown in a period T.sub.3. To sustain stable resonance, it is desirable to replace the energy consumed by the magnetron. Therefore, VG is applied to the IGBT gate to turn the IGBT on again to supply energy to the primary coil in a period T.sub.4. The characteristic of the resonance circuit is to reduce the switching loss by turning on the IGBT again at the point where the voltage V.sub.CE between the IGBT's collector and emitter falls to zero. FIG. 22C shows the resonance waveform of the primary coil voltage waveform V.sub.P.
V.sub.CE of the IGBT can be defined: EQU V.sub.CE =V.sub.DC -V.sub.P (Equation 4)
where V.sub.p is the voltage of the primary coil.
Accordingly, the waveform of V.sub.CE has a high voltage peak as shown in FIG. 22D due to the effect of resonance. The time T.sub.OFF during the resonance period T.sub.2 to T.sub.3 is determined by the capacitor, leakage transformer, rectifier connected to the secondary coil, circuit constant of the magnetron, and the quantity of energy provided to the leakage transformer in the circuit shown in FIG. 21. The period T3 in which V.sub.P .gtoreq.V.sub.DC is desirable for allowing V.sub.CE to fall to zero or below. The IGBT is turned on again in the period T.sub.4 to replace the energy consumed by the magnetron, thus allowing stable resonance to be established.
Energy provided to the leakage transformer is determined by the ON time T.sub.ON of the IGBT, and a shorter ON time T.sub.ON translates into a smaller quantity of power. The driving frequency f of IGBT can be defined: EQU f=1/(T.sub.ON +T.sub.OFF).
Since T.sub.OFF is mostly fixed, f rises as T.sub.ON becomes shorter, i.e. a smaller quantity of power.
V.sub.CE is given by Equation 4, and V.sub.CE does not become zero or below unless the relation V.sub.p .gtoreq.V.sub.DC is satisfied during the period T3. If power is reduced, the energy provided to the leakage transformer, i.e. energy powering the resonance, becomes smaller, and this relation may not be satisfied. This prevents the turning on of the IGBT at zero voltage, resulting in switching loss.
Furthermore, energy for resonance is also determined by the power supply voltage V.sub.DC according to Equations (2) and (3). Smaller voltage means lower energy, resulting in greater difficulty in satisfying the relation V.sub.P .gtoreq.V.sub.DC. This is the third disadvantage of the prior art.
A brief explanation of the magnetron is given next.
The magnetron is a vacuum tube for generating microwaves, and two conditions need to exist for the magnetron to be driven. The first is that the cathode temperature may need to be increased to about 2,100 K. The second condition is the application of a high negative voltage between the anode and cathode. To satisfy the first condition, current is supplied to the cathode from the tertiary coil of the leakage transformer to increase the cathode temperature. To satisfy the second condition, the high voltage output of the secondary coil of the leakage transformer is converted to DC at high voltage by the rectifier, and a high DC voltage is applied between the anode and cathode. The relation between the voltage VAC across the anode and cathode of the magnetron and the anode current IA when the cathode temperature is about 2,100 K is shown in FIG. 23.
VBM in FIG. 23 is called the starting voltage, and VBM of -3.8 kV is commonly used in microwave ovens for home use. The power P.sub.MG of the magnetron can be defined: EQU P.sub.MG =V.sub.AC I.sub.A (Equation 5),
about 70% of which is emitted in microwave form.
The frequency of generated microwaves is 2.45 GHz, but low levels of unwanted radio waves at other frequencies are also generated. To eliminate these, the magnetron may require a noise filter comprising a capacitor and a coil.
In the circuit diagram shown in FIG. 21, the tertiary coil of the leakage transformer is connected to the cathode of the magnetron. Power is controlled by the ON time of IGBT, and the ON time is shortened to reduce the power as described above. This reduces the voltage generated in the tertiary coil, resulting in a decrease in the current passing through the cathode. The frequency f also increases. The impedance Z.sub.L of the coil of the noise filter provided in the magnetron can be defined: EQU Z.sub.L =2 .pi.fL.sub.N (Equation 6)
where L.sub.L is the inductance of the coil of the noise filter.
Since the frequency f also rises, the cathode current is suppressed as the impedance becomes higher, leading to a further decrease in the cathode current. This is the fourth disadvantage of the single switching element voltage resonance circuit of the prior art.
The fifth disadvantage is associated with the starting of the magnetron.
The magnetron is not operable unless the cathode temperature reaches a temperature of around 2,100 K. On starting, a certain time is required for the cathode temperature to rise. Since one of the benefits of microwave ovens is high-speed cooking, it is important for the magnetron in a microwave oven to start up as quickly as possible. For this purpose, as large a current as possible is supplied to the cathode when starting, to generate a rapid increase in temperature. However, if a large current is supplied to the cathode on starting, the voltage of the secondary coil simultaneously increases because the tertiary coil for supplying the current to the cathode and the secondary coil for supplying high voltage to the magnetron are configured using a single leakage transformer. In addition, since the magnetron is started from low power by radically shortening the ON time of the IGBT, the impedance of the coil provided to the cathode of the magnetron rises to a high level, thus increasing the suppression of the cathode current. For supplying sufficient cathode current under these conditions, the voltage of the secondary coil may need to be further increased. FIG. 24 shows the change in characteristics with time of the voltage V.sub.AC between the anode and cathode from the starting point to the steady state in which the magnetron oscillates normally. During the time T.sub.S, the voltage across the secondary coil is high because a large current is supplied to the cathode. Then, after the time T.sub.S, the magnetron starts to operate, and V.sub.AC drops to V.sub.BM. V.sub.BM is about -3.8 kV, and the voltage generated on starting is about -7 kV. Accordingly, the withstand voltage of the diode and capacitor configuring the rectifier are desirably designed with this voltage taken into account. This is the fifth disadvantage.
Performance requirements for the power supply for driving the magnetron are described next. First, a high voltage may be desirable for driving the magnetron. Therefore, if any foreign substance such as dust attaches to the high voltage portion, sparks may be generated. If this happens, the operation of the circuit must be stopped immediately to avoid fire or smoke production by the components forming the power circuit due to continuous sparking.
In addition, since the magnetron is a vacuum tube, gas may be generated from the copper and tungsten of which it is comprised. If such gas is produced in a portion where the electric field is concentrated in the vacuum tube, sparks may occur inside the tube. If sparks occur, the impedance between the anode and cathode of the magnetron rapidly changes, and this may affect the operation of electric components such as the IGBT. Also, in this case, operation without causing failure of electric components may need to be assured. This is the second requirement.